Dark surface unchanged by light source changes
It seems from your render that the black/dark part is a reflection of the background (the environment). Try adding a large diffuse white plane above/in front of your objects so that there is something there to reflect instead of just the empty blackness of your world. Make this plane invisible to the camera or use the compositor to exclude it from the final render if it gets in the way of your scene (or move it far enough away so it isn't in view of the camera).